Moonpig, a large online seller of personalized greeting cards and gifts, shut down its mobile apps Tuesday because of a security weakness that could have given hackers access to customer information.

A developer named Paul Price found that Moonpig’s API (application programming interface), the online service used by the company’s mobile apps to interact with its website, lacked basic security features.

Price found that requests from Moonpig’s Android application to the API used a static set of credentials, regardless of customer account. The only thing that differentiated requests from different users was a customer ID included in the request URL.

Since the customer IDs were sequential and the API didn’t use authentication—at least not in a meaningful way—an attacker could send requests on behalf of all customers by iterating through different customer IDs, Price said.

According to U.K.-based PhotoBox Group, which owns Moonpig, the service has over 3.6 million active users in the U.K., Australia and the U.S.

“An attacker could easily place orders on other customers’ accounts, add/retrieve card information, view saved addresses, view orders and much more,” Prince said in a blog post Monday.

One API method called GetCreditCardDetails did not return the customer’s full credit card number, but did return the card’s last four digits, its expiration date and the owner’s name, according to Prince. Another method returned the customer’s name, address, country, email and other details.

The developer claims that he notified Moonpig of the security issue more than a year ago, in August 2013, but that the company dragged its feet. As a result, he decided to go public with the details Monday, saying the company has had “more than enough time” to fix the issue.

“It appears customer privacy is not a priority to Moonpig,” he said.

The company is currently looking into the issue and has shut down its apps as a precaution.

“We are aware of the claims made this morning regarding the security of customer data within our Apps,” Moonpig said on its corporate website. “We can assure our customers that all password and payment information is and has always been safe. The security of your shopping experience at Moonpig is extremely important to us and we are investigating the detail behind today’s report as a priority.”

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